The effect of cryogenic laser peening (CLP) on the microstructure of 2024-T351 aluminum alloy and its strengthening mechanism are investigated. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the microstructure evolution of 2024-T351 aluminum alloys. Results show that when compared with room-temperature laser peening (RT-LP), CLP has a more remarkable grain refinement effect and produces high-density dislocation. Additionally, CLP results in more and finer black spherical second phases, which are uniformly distributed throughout the samples and can be analyzed using EDS as the S phase (Al2CuMg). The in-depth microstructures of the samples treated by CLP are shown to have gradient distributions with different morphologies, and the microstructures of the matrix layer of these samples are superior to those obtained from samples subjected to RT-LP. There are two main strengthening mechanisms associated with CLP: one is that the cryogenic environment restrains the dynamic recovery of dislocations,reducing the thermal activation energy needed to promote the inhibitory effect of refined grains and second phases on dislocations; the other mechanism is related to the fact that the plastic deformation and internal stress caused by the volume shrinkage of samples under cryogenic environment can significantly produce structural strengthening effects.